Polyurethane elastomers provide unique performance in many engineering applications by combining elastomeric properties with high abrasion and tear resistance. One example is the PU elastomer used in the three main elements of a conveyor belt construction, i.e., as a coating for the belt's carcass, as a coating cast on rollers or idlers, and as a material from which to cast/mold conveyor belt scrapers. Originally natural rubber was used in these applications, but that has been superseded by polyvinylchloride (PVC), polyolefins, nitrile and silicone rubbers, and PU elastomers.
The advantage of PU technology in this application comes from its excellent physical properties not the least of which is its exceptional abrasion resistance. In the context of Shore A hardness, especially for a Shore A (ShA) hardness is excess of 65, natural rubber cannot compete with high performance PU elastomer in terms of performance and the life of the cast parts.
While fire retardance is important to many outdoor conveyor belt applications (to reduce the spread of fire in case of roller blockage and burn during the running of the conveyor belt), in underground coal mine applications antistatic properties are also required to prevent any electrical discharge and potential sparking that could be a source of fire or explosion. To meet these requirements PU elastomer are modified with additives responsible for flame retardance and antistatic properties.
Some of these additives may interact with catalyst packages used to make the PU elastomer. One of the most popular catalysts of low sensitivity to these type of additives is mercury-based THORCAT 535. However, mercury-based catalysts are disfavored due to the residual amounts of mercury in the final product that result from their use and as such, non-mercury catalysts are of interest to the manufacturers of PU elastomers. The same is true for lead-based catalysts. While non-mercury and non-lead based catalysts are available for PU elastomer production, many do not exhibit acceptable storage stability when admixed with polyol compositions that contain the additives that are necessary for the desired flame retardance and/or antistatic properties of the final PU elastomer with good mechanical properties acceptable processing parameters of gel or operating time and curing or molding time.